When is the Brain “Mature”?

Ask a handful of people at what age an individual becomes a
fully-formed adult and you are likely to receive just as many answers. Some
might say it happens when you get your first full-time job, others may say it
doesn’t occur until you become a parent. Another one or two might joke that
adulthood is the kind of thing that can remain elusive even after decades.
Others may simply invoke the rules defined by public policy and say “18 years
of age” or “when I could drink.”

But what happens when you ask a developmental neuroscientist
when the human brain reaches “maturity”?

“We’re
learning that there isn’t a one-size fits all message for when an individual
reaches maturity, nor a one-size fits all method for even how we should measure
maturity when it comes to the brain,” says Abigail Baird, a
developmental neuroscientist at Vassar College and a Dana Alliance for Brain
Initiatives (DABI) member.
“It would be great if, say, this one spot in the brain turns blue when you are
fully mature. But it doesn’t work like that. We’re learning that maturation is
about the refinement of circuits and larger networks that produce increasingly
coordinated behavior and brain activity. And those refinements and improvements
in neural coordination are heavily dependent on not only neurobiology, but also
practice and experience. And this is all without even considering the
influences of individual differences, which undoubtedly have a significant
impact in the nature and timing of maturation."

Despite the lack of a qualifying blue spot, new scientific
findings regarding the brain, adolescence, and neurodevelopment are informing
legislation and public policy across the country. Those results are shaping
legislation ranging from the age one can legally buy tobacco products to when
one might be incarcerated without the possibility of parole. And given that
policymakers are now paying close attention to the science, developmental
neuroscientists suggest it’s time we reconsider the concept of maturity when it
comes to the brain.

The Development
Argument

Historically, US society has considered one’s 18th
year to be the end of adolescence—and the doorway to official adulthood. It’s
about the time when most finish secondary schooling, as well as the average age
when one concludes physical body growth on the outside.

Certainly, adolescence itself is a time of great change in
the brain. Martha Denckla, DABI member and director of developmental cognitive
neurology at the Kennedy Krieger Institute at Johns Hopkins University, says
that many important processes occur during the teen years to help facilitate
vital neural circuits. Those processes include the reduction of cortical gray
matter; shifts in intrinsic patterns of connectivity; myelination of critical
circuits; and changes to metabolic activity, hormone levels, receptor density,
and neurotransmitter levels. Some of these changes occur before the age of
18—others do not resolve until long after.[i]

“We see that motor control, meaning the myelination of the
motor pathways, occurs around 15 on average. Then the dorsolateral prefrontal
cortex, responsible for cognitive control and executive function, is pretty
much myelinated by 25,” she says. “But then you start talking about emotions—and
everyone realizes the impact of emotions on cognitive control. They can change
how much control you have. So, when you look at the medial and orbital surfaces
of the frontal lobe, which some call the ‘social’ brain, the mean age of
myelination of those connections between the limbic system and those frontal
areas is about 32. That’s a far cry from 18.”

Context matters—and it matters a lot. Allan Reiss, a
pediatric psychiatrist at Stanford University and DABI member, says the
evidence shows that 18, neurobiologically speaking, is quite an arbitrary
number, especially now that we understand the brain changes in response to
environment, at age 18 or 82. Consequently, there is a discrepancy between
being a legal adult and a biological one.[ii]

“The brain is always in somewhat of a dynamic state. There
are certain developmental periods where it’s more dynamic than others, such as
the teenage years and young adulthood,” he says. “But you can’t say it gets to
a certain point and is mature. The brain continues to mature in different ways
throughout your life. And whether a brain is mature, or finished with those
dynamic states of development you see in adolescence, may not be the right
question—especially if we are considering society and public policy. Perhaps we
should be asking a different question, which is, ‘What is the average age at
which human beings are likely to make rational decisions about important events
in their lives?’” And the answer is going to be, ‘It depends.’ It depends on
the person, the kind of decision you are trying to make, and what’s happening
around you when you are trying to make that decision. Context is important. All
those factors play a role and influence how well you can make those decisions.”[iii]

Legal Implications

While most hold 18 as the age of maturity in the US, Sarah
Bryer, executive director of the National Juvenile Justice Network, says that
the standard for adulthood varies from state to state, and policy to policy.

“There is no consensus around 18 on any front. There are
variations around when we consider young people able to make decisions about
whether they can drink, serve in the military, vote, consent to have
intercourse,” she says. “We are uneven in our application of when we consider
young people to be mature.”

She offers that juvenile courts have only existed since 1899
in the US.[iv]
Throughout the last century, the justice system’s treatment of children was
informed by existing, and she says “sometimes puritanical” beliefs about
childhood development. But evolving psychological research on the nature of
childhood and adolescence has helped shift the tenor of these courts. Yet,
until recently, modern brain science studies didn’t have much influence on how
minors were considered by law and public policy mandates.

Photo Credit: Shutterstock

Nathalie Gilfoyle, former general counsel for the American
Psychological Association (APA), says that neuroscience came to the forefront
in legal proceedings with the landmark Supreme Court case, Roper v. Simmons,
a case that determined capital punishment was unconstitutional for offenders
under the age of 18[v]. That
decision was made, she says, due to the converging evidence suggesting that
executive control does not fully develop until the mid-20’s.

But what to do with those findings within the legal and
public policy spheres is somewhat of a conundrum. Gilfoyle says that the body
of research that helped sway the court in Roper v. Simmons now also has
been deemed relevant in cases regarding competence to stand trial, waive
Miranda rights, and assist counsel in legal proceedings—often pushing up ages
in some states and jurisdictions to 18 years. But these findings have been
contrasted with other cases involving juvenile decision-making like Hodgson
v. Minnesota, a 1990 Supreme Court case about parental notification and
abortion.[vi]

“In Hodgson, the APA took the position that parental
notification was not a necessary component for those under 18 seeking an
abortion. By citing research about cognitive decision-making, the APA noted
that teenagers were mature enough to make medical decisions on their own, in
consultation with medical advisors,” she explains. “In his dissent in Roper v. Simmons, Justice [Antonin]
Scalia took the position that there was an inconsistency between our positions
in Hodgson and Roper. He called it a flip flop. But there is a significant
difference between cognitive development research that is relevant to medical
decision-making and the social science research related to bad decisions by
juvenile criminal defendants and the ability to predict adult character.”

This is why we might do better to stop using the term
maturity altogether when we’re discussing brain development, says B.J. Casey,
DABI member and director of the Fundamentals of the Adolescent Brain (FAB)
Lab at Yale University.

“By using this term of maturity, we’re setting ourselves up.
We seem to be saying that there’s one single point in time at which we’re able
to do everything well,” she says. “Even if the brain isn’t fully mature, people
can still make good decisions. Part of the problem with how to apply brain
research is that the results are often oversimplified, which is why some people
might call something like this a ‘flip flop.’ But the truth is there are some
situations where teens make good decisions and others in which they don’t. And
it’s important that we better understand them so we can make effective policy
decisions without it sounding like we’re trying to use the data one way or the
other.”

Bridging Science and
Policy

So how can science be used to benefit the greater whole of
society, and encourage adolescents to become productive and, yes, mature
members of society? The key, Casey says, is for scientists and lawmakers to
work closely together to ensure findings are interpreted and applied
accurately. Richard Bonnie, director of the Institute of Law, Psychiatry, and
Public Policy at the University of Virginia, says that using the science to
direct policy isn’t an impossibility—and it doesn’t have to be complicated once
you disconnect from the idea that there is a single age of maturity.[vii]
He argues there is “too much distance” between what we now understand about the
neurobiological nature of brain development and our current set of age-governed
laws and public policies.

“Even when we hold young people responsible for their
behavior, quite often they are subjected to a punishment that focuses more on
the offense rather than their abilities and characteristics,” he says. For
example, in many cases, a charge of murder makes it more likely that a teen
will be tried as an adult—despite the particulars of the crime or the
perpetrator’s background. Because of this, Bonnie argues, what we’re learning
about the science of neurodevelopment has strong potential implications
regarding our policies on punishment—and how we might do better to reform the
criminal justice system to balance the need to protect society and rehabilitate
youthful offenders. He says that it pays to consider both cognitive capacities
and circumstances—but it still is important to hold individuals under 18 accountable
for their behavior.

“Accountability is important developmentally, too—part of
successful development is learning to take responsibility for your behavior—so
that also needs to be considered,” he says. “We have an opportunity to use the
science to figure out how we can best help offending individuals become
productive members of society.”[viii]

Bryer agrees—and hopes what we are learning about brain
development will help reform the juvenile justice system away from punishment
and toward more rehabilitation-oriented policies.

“The research around brain development has been important to
help policy makers think through what outcomes they are trying to actually
achieve within the system,” she says. “It opens up so many avenues of policy
and reform. It changes how we think about connections—not just those in the
brain, but those to the family, to the community, and around education. It
helps us rethink, if young people are incarcerated in a facility, what should
that facility look like? How can we provide the support they need to ultimately
be successful? We need to figure these things out so we’re not wasting money or
opportunities to help improve society overall.”